Optical touch button switches are non-contact, zero-pressure replacements for mechanical push button switches used in applications where an operator must repeatedly actuate a device, sometimes thousands of times per shift. Optical touch button switches are typically used in an industrial environment on machines such as punch presses, press brakes, shears, riveting machines and molding presses. Industrial machines including mechanical push buttons require an operator to overcome the spring-return force of the button, which has been said to cause repetitive stress disorders such as carpal tunnel syndrome.
Optical touch button switches typically consist of a U-shaped housing in which a beam of light is projected across the gap. When a finger or hand is placed in the gap, the beam is broken and the control function is initiated. An example of an optical touch button switch is disclosed in U.S. Pat. No. 4,939,358. The problem with optical touch button switches such as that disclosed in U.S. Pat. No. 4,939,358, is that such switches can be triggered unintentionally. Typically, false triggering of optical switches is caused by an operator sleeve, insects or dust breaking the light beam projected across the gap of the U-shaped housing. Also, in spite of the safety precautions, some operators will attempt to trick the switches by placing an object in the switch opening to trip the optical switch.
Since one application for an optical touch button switch is to operate heavy machinery in an industrial environment (such as punch presses), various regulatory requirements exist to assure that the machinery will not be inadvertently operated. One of the requirements is that two initiating devices be used in a configuration that requires the operator to place both hands on the initiating devices in order to initiate the machine function. Further, the requirements state that both hands must be placed on the initiating devices simultaneously and that, if either hand is removed, the machine must be immediately stopped.
Various ancillary control circuits have been developed to meet these regulatory requirements, and they are generally referred to as anti-tie-down circuits. The function of an anti-tie-down circuit is to prevent the operator from tying down one of the actuating devices in order to have a free hand to insert work pieces into dangerous areas on the machine. These anti-tie-down circuits are themselves subject to various regulatory requirements to assure that failures within the anti-tie-down circuit do not cause inadvertent operation of the machine. In fact, the reliability of the anti-tie-down circuit is of more significance than the reliability of the actuating devices, because if properly designed, the anti-tie-down circuit requires simultaneous operation of both actuating devices. Thus, if one actuating device false operates, no action is taken, and the probability of both actuating devices false operating simultaneously is low. Notwithstanding the reliability of anti tie-down circuitry in a general application, there is always a desire to improve the reliability of the entire system. Thus, attention is paid to the design of the initiating devices, particularly when they are electronic rather than mechanical. Previous systems such as those disclosed in U.S. Pat. No. 5,410,148 and U.S. Pat. No. 5,077,467 have attempted to provide redundancy and fault monitoring. However, these systems have not provided a solution that can pass a failure mode effects analysis. The primary reason the solutions provided by the above referenced prior systems could not pass a failure mode effects analysis is that the monitoring circuits that monitor internal device functions are not control reliable. There is a need for an optical switch having internal device monitoring functions that has a second monitoring circuit that monitors the internal device monitoring circuit. Such a system is needed so that component failure in the internal monitoring circuit does not cause false actuation of a machine, but provides information to a separate controller that prevents the machine from operating until the detected failure within the monitoring circuit is repaired.
There are a number of failure mode possibilities in optical touch button or capacitive touch plate switches that can lead to false actuation. For example, a light source or photo detector failure will, in most cases, result in a dark condition, just as if a hand or finger had been placed in the gap of the U-shaped housing. There are also many failure modes of electronic components that drive the light source which could cause a dark condition. There are also many failure modes of electronic components that amplify the photo detector signal that could cause a dark condition. U.S. Pat. No. 5,077,467 attempts to solve these problems by monitoring the light emitting diode and the driver transistor adapted to control a mechanical relay. However, there is no attempt to monitor the monitoring circuitry to determine if failures occur in that circuitry. There is a need for a system to provide assurance that all of the electrical components that comprise the sensing system are operational in the dark condition. Such a system would need to actively monitor the circuitry controlling the light emitting and light receiving functions as well as the circuitry monitoring the light emitting and light receiving circuitry. In such a system, failure of any components can be detected so that false triggering of the switch or continued actuation of the industrial machine after a hand or finger is removed from the housing of the switch does not occur.